In this electronic era a great variety of direct current power supplies have been designed to meet a vast spectrum of differing circuit requirements. Depending on the amount of power to be supplied and the amount of regulation of voltage output required, these designs have ranged from relatively simple and inexpensive combinations of two or more diodes forming a full wave rectifier to very complex and expensive designs employing large power transistors and integrated circuits. An informative summary of commercially available versions of such supplies can be found in the June 16, 1981 issue of Electronics magazine, beginning at page 106.
For some applications, such as some key telephone systems, there has been a need for an inexpensive, energy efficient power supply, the output voltage of which need not be precisely regulated but which should not be allowed to rise above a predetermined limit regardless of input line voltage variations. Conventional switching supplies which regulate power through pulse width modulation are considered too costly for such an application, and conventional linear supplies tend to be too inefficient, thus requiring provision for dissipating excessive amounts of heat.